1. Field of the Invention
The present invention relates to color printing having a highlight color image mapped from a full color image and, more particularly, to highlight color printing which preserves information important to the viewer.
2. Description of the Related Art
Color images are a significant element in today's printing industry. As a result, electronic color printers and color image creation tools have been increasingly developed to obtain color images using electronic printing methods. Much of the color printing is performed using full color, the gamut of colors including tints and shades of the full color spectrum--reds, greens, blues and their combinations. A significant amount of color printing, however, can be performed using a highlight color. In this type of printing, only two inks are used in the printing process. These inks comprise black and a highlight color (usually red or blue). Electronic printers may be designed specifically for highlight color printing. The highlight color printer is generally faster and less expensive than the full color printer since only two inks are processed as opposed to the three or four inks which must be processed in order to obtain full color images.
The gamut of full colors is a three-dimensional volume which can be represented by the double hexagonal cone 10 illustrated in FIG. 1. In this representation, shades vary from dark to light as one moves upwards vertically. Tints vary from unsaturated grays to fully saturated colors as one moves out radially. Hues vary as one moves angularly in the horizontal plane.
The gamut of colors available to a highlight printer can be represented by the two-dimensional triangle 12 illustrated in FIG. 2. This is a slice from the full color double hexagonal cone of FIG. 1 at the angle of the highlight hue.
Prior attempts to print a full color image on a highlight color printer involve mapping the three-dimensional double hexagonal cone of FIG. 1 to the two-dimensional triangle of FIG. 2. The printer makes its best effort to render the highlight color image by mapping the full color specification into the set of colors which it can produce. In such a mapping, many different colors in the full color space will be mapped to the same color in the highlight color space. Information important to the viewer is often lost.
The related art has disclosed printing systems which attempt to provide color images.
Harrington U.S. Pat. No. 4,903,048 discloses color imaging using ink pattern designs in conjunction with registered two-color imaging to form simulated color images. A printing apparatus is described which is used to perform the two-color imaging.
Edwards U.S. Pat. No. 4,554,241 discloses a method of printing a realistic image of an original on a sheet. Two printing plates are used to print different impressions with two different coloring media.
Cole et al. U.S. Pat. No. 4,636,839 discloses a method and apparatus for generating color matte signals. RGB color components are calculated from hue, saturation and luminance value inputs. A new luminance value Y and color difference components CR and CB are calculated from these RGB values. Various algorithms are disclosed which are used for color calculation.
McManus et al. U.S. Pat. No. 4,670,780 discloses a method for matching hard copy colors to display colors for registered ink jet copiers. A color transformation for matching hard copy color to display color consists of an MSW color space which restricts the hard copy color to certain percentages of binary mixtures of inks (M), single inks (S) and paper white (W). Color data in XYZ is converted to the MSW space and unreachable display colors are matched into reachable hard copy colors. Corrections for color shifts due to interactions of the inks are also disclosed.
Langdon U.S. Pat. No. 4,761,669 discloses an electrophotographic highlight color printing machine in which printing is done in at least two different colors. Methods for transferring multiple color images simultaneously are disclosed.
Sasaki et al. U.S. Pat. No. 4,682,186 discloses a method for forming a color image. The color image is formed by using a plurality of coloring materials and controlling quantities of the coloring materials. When a density to be reproduced exceeds the density reproducible for that coloring material, the quantities of other coloring materials are reduced.
Hasebe U.S. Pat. No. 4,907,078 discloses a method of reproducing color images wherein two charge-coupled devices of different wavelengths are used to scan a full color document. The outputs of the charge-coupled devices are fed into a lookup table to determine the appropriate color in a two-color system. The output from the table is then recorded on a piece of paper. The system can be used for any type of copier.
Davis U.S. Pat. No. 4,894,665 discloses a method of generating an expanded color set of a low resolution color printer wherein a four-color printer can be expanded to twelve colors by printing a black dot next to a line to make the line seem darker. Two algorithms are provided for determining whether or not a line is critical and for enhancing a line.
Iwata U.S. Pat. No. 4,908,779 discloses a display pattern processing apparatus wherein a system can be programmed to convert a full color image into a number of other formats. An example is shown wherein an RGB image is converted into a two-color image.
While the related art attempts to map a full color image to a highlight color image, it does not recognize that certain information from the full color image should be preserved depending upon how the color is being used and what type of image is being created. Accordingly, information is lost in these devices.